Abstract

The possibility of dating minerals by the accumulation of ^4He from U and Th decay
has been recognized for many years (e.g., Strutt 1905), but in the century since the idea
was first conceived, the method has rarely been applied successfully. After several
investigations of (U-Th)/He dating of various minerals (e.g., Damon and Kulp 1957;
Fanale and Kulp 1962; Damon and Green 1963; Turekian et al. 1970; Bender 1973;
Leventhal 1975; Ferreira et al. 1975) the technique was essentially abandoned as yielding
unreliable and usually low ages, presumably as a result of diffusive He loss possibly
associated with radiation damage. In 1987, Zeitler and coworkers rekindled interest in the
method by proposing that in the case of apatite, He ages might be meaningfully
interpreted as ages of cooling through very low temperatures. Laboratory diffusion data
presented by these authors indicated a closure temperature of about 100ºC, a value
supported by more recent studies (Lippolt et al. 1994; Wolf et al. 1996b; Warnock et al.
1997). Consistent with this interpretation Wolf et al. (1996a) found that apatite He ages
increase systematically with sample elevation in a mountain range, as expected for
exhumation-induced cooling through a low closure temperature. Based on the strength of
these results and additional laboratory (Farley 2000) and natural (Warnock et al. 1997;
House et al. 1999; Stockli et al. 2000) constraints on He diffusivity, recent attention has
focused on applications of apatite He thermochronometry. There is also renewed interest
in He dating of other U- and Th-bearing minerals both for dating mineral formation and
for thermochronometry. For example, Lippolt and coworkers have undertaken detailed
studies of He diffusion and dating of various phases, most notably hematite formed in
hydrothermal systems (Lippolt and Weigel 1988; Wernicke and Lippolt 1992; Lippolt et
al. 1993; Wernicke and Lippolt 1994a,b).
Here I present an overview of recent techniques, calibrations, and applications of the
(U-Th)/He dating method; Hurley (1954) provides an excellent summary of earlier work in
this field. Much of this paper focuses on apatite, because the He behavior and requisite
analytical techniques are better established for this phase than for other target minerals,
such as zircon and titanite. Similarly, much of this paper concerns He diffusivity behavior
required for thermochronometric applications, yet recent work is also considering applications
to direct dating, for example, of young tephras (Farley et al. 2001).